Optical communications networks are used to transport large amounts of voice and data through fiber optic cables. Wave-division multiplexing (WDM) can be used to transmit multiple channels of optical information through the same fiber optic cable. Multiple channels are carried in the same fiber by assigning different optical wavelengths to each channel.
The optical signals degrade over distance such that re-generation or re-amplification may be required. Moreover, the channels within a fiber tend to degrade at different rates. The total mean output power is frequently used as an indicator of signal degradation. Optically amplifying the signals to maintain the total mean output power at a constant is not particularly effective due to the differences in degradation between channels. Moreover, increasing the amplification gain to maintain a constant mean output power without regard to individual channels can create significant distortions on an individual channel basis. Increasing gain to compensate for the loss of a few channels may result in too much amplification for the remaining channels individually even though the total mean power output remains constant. Accordingly, optical power is determined and regulated on a channel-by-channel basis.
In order to measure optical power on a channel-by-channel basis, an identifying pilot tone is superimposed upon each channel. Within a given fiber the pilot tones should be unique in order to distinguish all channels for the purpose of power calculations. Given that the pilot tone and the channel signal originate from the same source, the optical channel power can be inferred from the pilot tone power and its fixed modulation depth at the source. The optical channel power can then be used to adequately control the optical amplifiers in the network.
This technique helps to regulate the quality of optical signals along a known optical path, but the technique does not work if there are network configuration errors such as an identification of multiple sources for the same optical signal or a complete loss of a selected channel. Generally, differences between the planned network configuration and the actual network configuration can significantly impair the process of managing the network.